In a numerical control device (CNC), a workpiece is machined to a desired contour by moving a tool at a commanded speed along a commanded path by a machining program.
To obtain a satisfactorily machined surface corresponding faithfully to a command by a machine tool using such a numerical control device, it is essential to use a servo mechanism with a quick response that enables it to follow rapid changes in the command, and having a high stability by which a stable movement without vibration is ensured.
A speed detector and a position detector of a servomotor in the servo mechanism detect speed and position and a desired control is carried out by a feedback of the information obtained to a control circuit. The servo mechanism can be classified into three types; a semi-closed loop, a closed loop and a hybrid servo method, according to the method used to detect a position.
The best method among the above three servo mechanism methods is adopted for each machine tool, according to the accuracy and rigidity, etc. required by the machine tool.
Nevertheless, the mechanical rigidities of the axes of general machine tools are different from each other, and these differences are particularly noticeable among large-sized machine tools. Therefore, when a cutting is carried out by a machine tool having more than two axes, form errors are generated in the vicinity of a starting point and an end point of machining or at corners, and thus the accuracy of the cutting work becomes poor.
FIG. 6 shows a state of cutting with two axes according to a conventional method. In this Fig, it is assumed that machine rigidity is relatively weak at the X-axis and relatively strong at the Y-axis, and further, that the X-axis is moved by 0.100 mm and the Y-axis by 0.200 mm by these control axes.
If the machine rigidities of the X-axis and the Y-axis are the same, a cutting in a straight line, without error, as shown by the straight line A, is usually obtained, but if the machine rigidity of the X-axis is weak, the movement of the X-axis is too little, as shown by the curve B1, and even though the ratio of distribution pulses of the X-axis to the Y-axis is 1:2, the actual ratio of the machine positions becomes 1:4, as shown by the point C, which results in a form error.